Field of the Invention
The present invention relates to a display device, and more particularly, to an organic light emitting diode (OLED) display device having improved reliability by preventing defective bonding between a substrate and an encapsulation substrate.
Discussion of the Related Art
An OLED display device displays an image by adjusting an amount of light emitted from a light emitting layer. The OLED display device also have a reduced weight and volume compared to the larger cathode ray tubes (CRTs).
Further, the OLED display device is a self-emitting device using a thin light emitting layer disposed between electrodes. In more detail, the OLED includes a first electrode functioning as an anode and being connected to a thin film transistor disposed in each sub-pixel region of a substrate, a light emitting layer (EML), and a second electrode functioning as a cathode.
When a voltage is applied to the first and second electrodes of the OLED, holes and electrons are recombined in the light emitting layer to create excitons that emit light when falling from an excited state to a ground state.
However, such OLED easily deteriorates from external factors such as moisture, oxygen, UV radiation, and manufacturing conditions of the display device. In the related art OLED display devices, a substrate including the OLED and an encapsulation substrate are bonded to each other via an adhesive layer.
In more detail, FIG. 1 is a cross-sectional view illustrating defective bonding of a related art OLED display device. Referring to FIG. 1, an OLED 11 is formed on a substrate 10, and the substrate 10 is bonded to an encapsulation substrate 15 via an adhesive layer 14. For example, the adhesive layer 14 is applied to the encapsulation substrate 15, and then a cover peeling layer is removed from the adhesive layer 14 (i.e., to expose the adhesive layer 14). The encapsulation substrate 15 with the attached adhesive layer 14 is pressed or combined with the substrate 10 including the OLED 11.
However, during this joining or combining process, a step difference between a display region including OLED 11 and a non-display region not including OLED 11 is formed. That is, as show in FIG. 1, a distance D2 between the substrate 10 and the encapsulation substrate 15 in the non-display region is greater than a distance D1 between the substrate 10 and the encapsulation substrate 15 in the display region. Thus, moisture, oxygen, or other particles can enter this region and damage the OLED 11.